This application claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 199 19 716.4, filed on Apr. 30, 1999, through the PCT International Application PCT/EP00/03140.
The present invention relates to a microelectronic package including an electronic component which is attached to a substrate element by an attachment layer comprising an adhesive and spherical spacer elements provided in the adhesive.
It is generally known to use approximately spherical particles as spacers in a glued joint between two microelectronic packages. For example, German Patent Application Laying-Open Publication 2,756,500 discloses such an arrangement in which the spherical particles have a diameter equal to the desired stand-off, and a multitude of them are distributed in the adhesive layer to form the spacer means.
In German Utility Model Publication 91 16 206, spherical bodies are used as spacer elements between insulating glass plates. The spacer elements are plastically deformed superficially to form flattened areas under the effect of increased pressure, as a result of which the manufacturing tolerances of the spherical spacer elements can be reduced to a set stand-off by pressing.
Furthermore, in practice, packages with force-sensitive sensor elements, for example acceleration sensors for automotive vehicles, are attached to a substrate element, for example a printed circuit board, by adhesively bonding them together using an adhesive with such spherical spacer elements dispersed therein. Previously, glass balls have been used as spherical spacer elements for this purpose, because they are electrostatically insensitive, are relatively easy to mix into the adhesive, can be distributed in it, and are very cheap and readily available. However, serious disadvantages arise with the use of such glass ball spacer elements. Due to the cross-linking of the adhesive layer at a defined hardening temperature lying above the operating temperature range of the application, measurable distortions between the component and the adhesive layer are caused on the sensor element. Such distortions result, or are exacerbated because the glass balls have a significantly different elasticity and thermal expansion characteristic in comparison to the curing adhesive. As a result, these distortions lead to an additional shifting of the electrical zero point of the component. This must be compensated by an offset for the variable to be detected. However, despite this offset compensation, other measuring inaccuracies commonly occur, particularly if the component had only been attached by one drop of adhesive.
For example, the problem of mechanical distortion under temperature changes in the environment of a sensor is also known from U.S. Pat. No. 4,295,117. A base plate is selected with approximately the same coefficient of thermal expansion as that of the sensor chip. A pedestal and a substrate die are bonded to each other and to the chip by an elastic adhesive, and are arranged between the base plate and the chip so that the chip is isolated from the distortions of the base plate. Nevertheless, unwanted distortions can occur even within this bond when the adhesive compensates for the differing changes in length.
An electronic package, for example a BGA, is further known from the PCT International Publication WO 97/22993, in which the coefficients of thermal expansion of the spacer elements are matched to the coefficients of thermal expansion of the two components which are to be bonded to them. Generally, the spacer elements have a coefficient of thermal expansion which is significantly less than, and especially not greater than that of the adhesive. Glass (SiO2) or corundum (aluminum oxide Al2O3) are proposed as materials for the spacer elements. These materials are rigid and have a coefficient of thermal expansion significantly less than that of the adhesive, e.g. these materials have a thermal expansion coefficient much less than one tenth of that of the adhesive or typically about one hundredth of that of the adhesive or even less.
In view of the above, it is an object of the invention to provide a microelectronic package with an attachment layer including spacer elements in an adhesive, whereby mechanical distortions resulting from different thermal expansion characteristics of the various components can be avoided or minimized, and wherein the overall attachment layer provides an improved resilient compensation of any expansion differences or the like between the electronic component and the substrate on which it is attached. Another object of the invention is to reduce the manufacturing and operating tolerances and to simplify the assembly. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification.
The above objects have been achieved according to the invention in a microelectronic package comprising a microelectronic component attached to a substrate by an attachment layer including an adhesive and a plurality of approximately spherical, spheroidal or ball-shaped spacer elements. The nominal diameter of the spacer elements at any given temperature corresponds to or essentially determines a specified stand-off distance between the mounting surface of the microelectronic component and the facing surface of the substrate. The individual spacer elements do not necessarily all have exactly the same diameter corresponding to the nominal diameter, but instead have actual diameters within a typical plus/minus range around the stated nominal diameter, for example corresponding to the nominal diameter xc2x15%. The nominal diameter may be an average diameter or a maximum diameter determined by sieve grading or the like.
The spacer elements comprise a plastic material that is at least somewhat elastically flexible, e.g. being more easily elastically deformed than prior art glass or corundum balls used as spacer elements, or more easily elastically deformed than the adjoining substrate or electronic component surfaces. The plastic material may be a single polymer of a single type of monomer, or may be a mixture or copolymer of plural different polymers. The spacer elements preferably essentially consist of the above described plastic material.
The plastic material is preferably selected so that the coefficient of thermal expansion of the spacer elements approximately corresponds to the coefficient of thermal expansion of the adhesive. This means, for example, that the coefficient of thermal expansion of the spacer elements is closer to the coefficient of thermal expansion of the adhesive, than is the coefficient of thermal expansion of the microelectronic component or the substrate. This further means, for example, that the coefficient of thermal expansion of the spacer elements is closer to that of the adhesive than is the thermal expansion coefficient of glass or corundum. More particularly, this further means that the coefficient of thermal expansion of the spacer elements is less than that of the adhesive by a factor of not more than 10, i.e. the coefficient of thermal expansion of the spacer elements is at least one tenth of (but preferably not more than) the coefficient of thermal expansion of the adhesive (CTEspacerxe2x89xa70.1xc3x97CTEadhesive).
A microelectronic package according to the invention is further developed in comparison to the prior art, so that it has lower tolerances and can be easily assembled.
A temperature-dependent distortion of the microelectronic component such as a sensor has been determined to be the cause of the above-discussed measuring inaccuracies, which arise from the very different coefficients of thermal expansion of the adhesive on the one hand and the spherical spacer elements on the other hand in the prior art arrangements. This gave rise to a shear effect between the glass balls and the mounting surface of the electronic component when the adhesive layer contracted at low temperatures. If this is compensated by an offset for a normal temperature, then at higher temperatures, and thus a lower shear effect, deviations occur once again.
Using spacer elements according to the present invention, with a coefficient of thermal expansion approximately corresponding to the coefficient of thermal expansion of the adhesive, i.e. at least significantly nearer than that of the prior art glass balls, has achieved a significant improvement in the manufacturing yield and a lower tolerance range.
Plastic balls or ball-shaped spacer elements having a generally spherical or spheroidal shape, whose coefficient of thermal expansion can be relatively well controlled, are preferably used for this purpose according to the invention. Plastic balls admittedly have a lower density and a tendency toward electrostatic charging, however this can already be compensated for by appropriate steps (e.g. adding any conventional anti-static agent) during the mixing of the adhesive and the spacer elements. The particular advantage of the plastic, apart from the approximation of the coefficient of thermal expansion of the adhesive, is its elasticity and elastic deformability, which have enabled the shear effect to be reduced still further.
Particularly, the coefficient of thermal expansion of the spacer element is less than that of the adhesive by a factor of no more than 10. Thereby, an adequate approximation of the coefficient of thermal expansion of the adhesive and a significant improvement of the temperature and manufacturing tolerances could be established.